Macrophages play a significant role in HIV-1, in both productive infection and maintenance of the viral reservoir. While the current efforts of eradication of HIV are mostly directed toward latently infected T-cells, macrophages are largely ignored. Macrophages are however likely to contribute to maintenance of the viral reservoir especially in the CNS where HIV infection is often associated with CD163 positive macrophages. Our previous work has demonstrated that substance P (SP), the endogenous neurokinin 1 receptor (NK1R) agonist, promotes HIV infection in macrophages, whereas treatment of macrophages with NK1R antagonists inhibits HIV infection. During the course of our current investigations, we made a novel observation which is the basis of this R21 grant; the treatment of human macrophages with SP ex vivo increases levels of both soluble and membrane bound forms of CD163, the scavenger receptor for haptoglobin-hemoglobin. CD163 is a marker which is uniquely expressed on cells of monocyte-macrophage origin and present in biological fluids in a stable soluble form (sCD163). Significant evidence has been accumulated demonstrating increases in CD163 levels in HIV infected individuals, however, the direct involvement of this molecule in HIV pathogenesis is unknown. Using two independent methods, cell sorting and siRNA, we demonstrated that higher levels of membrane bound CD163 are associated with higher HIV infection in macrophages. In the next phase of our investigations, we have designed a series of novel experiments in order to establish the roles of soluble and membrane-bound CD163 in HIV infection (Aims 1 and 2), and to determine if SP-induced enhancement of HIV infection is related to NK1R-mediated regulation of CD163 expression in human monocytes-macrophages (Aim 3). This R21 focuses on the mechanisms of HIV infection in macrophages and has direct implications for development of novel HIV therapies.